The present invention relates generally to ultrasonic vibratory welding apparatus and process and, more particularly, to such apparatus and process wherein prior to welding the workpieces to be welded together and the welding tip and anvil are pre-heated and, further, wherein the temperature of the welding tip and/or the anvil are monitored and controlled during the welding operation to insure that subsequent welds are generally uniform in size, shape and strength. Pre-heating of the tip, anvil and workpieces has been found to result in substantially stronger welds.
Ultrasonic vibratory spot welding processes for joining together two or more similar or dissimilar materials have been used for a number of years. Until recently, however, such methods were limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important. This limitation was due, in large measure, to the problems associated with the ultrasonic welding methods employed, most of which were in prototype stages. In those instances when weld strength and weld integrity were important, i.e., when joining together structural aircraft panels and the like, resistance spot welding procedures were used.
Ultrasonic spot welding procedures have recently demonstrated strong potential for improved sheet metal assembly at reduced cost when compared with resistance spot welding and adhesive bonding techniques. Early studies have indicated that welds effected using prototype ultrasonic welding equipment such as, for example, a Sonobond M-8000 ultrasonic spot welder, were superior to welds produced using conventional resistance spot welding procedures. These early trials indicated that for virtually any material combination, an ultrasonically produced spot weld has an ultimate yield strength of more than 2.5 times that of a weld produced using resistance spot welding equipment. Further tests indicated that ultrasonically produces spot welding can be accomplished with a 75% time and cost savings over conventional adhesive bonding techniques. Until now, however, ultrasonic spot welding for large structural metal parts was not possible in a production environment because of the numerous problems associated with the procedures.
Ultrasonic vibratory welding is a metallurgical joining technique which utilizes high frequency vibrations to disrupt the surface films and oxides and which, therefore, promotes interatomic diffusion and plastic flow between the surfaces in contact without any melting of the materials. Briefly stated, the ultrasonic welding process consists of clamping or otherwise securing together the workpieces under moderate pressure between the welding tip and a support anvil and then introducing high frequency vibratory energy into the pieces for a relatively short period of time, i.e., from a fraction of a second to a number of seconds. In many instances, the pieces to be welded may also be adhesively bonded together by the insertion of an adhesive bonding agent between the juxtaposed pieces before which result in a high strength, uniform bond between them.
One example of an ultrasonic spot welder particularly adapted for use on structural metal workpieces is the Sonobond Model M-8000 Ultrasonic Spot Welder marketed by the Sonobond Corporation of West Chester, Pa. This welder includes a transistorized, solid state frequency converter which raises standard 60 Hz electrical line frequency to 15-40 kHz and then amplifies the output. The high frequency electrical power travels through a lightweight cable to a transducer in the welding head where it is converted to vibratory power at the same frequency. The vibratory power is, thereupon, transmitted through an acoustic coupling system to the welding tip and then through the tip into and through the workpieces, with the vibratory energy effecting the weld.
The Sonobond M-8000 Ultrasonic Spot Welder includes a wedge-reed, transducer coupling system which transmits lateral vibrations of a perpendicular reed member attached to it so that the welding tip at the upper end of the reed executes shear vibrations on the surface of the workpieces. The transducer includes piezoelectric ceramic elements encased in a tension shell assembly and operates at a nominal frequency of 15 kHz. A solid state frequency converter with a transistorized hybrid junction amplifier powers the welder. The converter operates at a nominal frequency of 15 kHz with a power output variable up to about 4000 RMS RF watts. The welder may be turned to a precise operating frequency. The frequency converter includes a wide-band RF power measuring circuit which samples output power and detects forward power and load power based on the principle of bi-directional coupling in a transmission line. The signal is processed electronically to provide true RMS values which are selectively displayed on an LED panel meter as either the forward or load power. Forward power is the output of the frequency converter delivered to the transducer in the welding head while load power is the transducer drive power acoustically absorbed in the work zone. The difference between the two readings is the reflected power induced by the load impedance mismatch and is minimized during the welding operation by impedance matching techniques.
In early trials using prototype ultrasonic welding equipment to weld aluminum alloys, it was discovered that certain problems were encountered particularly with regard to "tip walking" and workpiece movement, i.e., the welding tip tended to move laterally or horizontally during the course of the welding cycle and the workpieces were sometimes extruded from the working zone. It was discovered that tip walking occurred because the workpieces located beneath the welding tip were in a mechanically unstable condition during the welding operation. As can readily be appreciated, tip walking and workpiece motion create a highly undesirable condition in that it is virtually impossible to precisely pinpoint the point of weld. Additionally, these conditions result in a lesser strength weld. In general, when collar clamps are applied tip walking might occur and when clamps are not used or are used too lightly then the workpiece will be spit out of the work zone and made to move.
During these trials, it was found that, during the course of the welding operation, the temperature of the welding tip and the anvil were particularly important since it was found that the quality and size of the welds effected were dependent upon such temperatures. During start-up when the welding tip and anvil were "cold", it was frequently impossible to effect a good weld. Oftentimes it would take six or seven "practice" welds before the equipment was hot enough to effect a good weld. Then, after about 30 or 40 welds, the welding tip and anvil became so hot that a cooling medium had to be passed over the parts to lower their temperature and insure good weld uniformity. It was found that when the welding tip and anvil "overheated", the resultant weld size would grow and thus cause non-uniformity in weld size, shape and strength.
Against the foregoing background of the invention, it is a primary object of the present invention to provide an ultrasonic welding method which is adaptable for use in the production welding of structural metallic parts.
It is another object of the present invention to provide such a method which results in high strength and durable welds.
It is yet another object of the present invention to provide such a method which produces uniform welds during the entire welding period.
It is yet still another object of the present invention to provide such a method which continuously monitors the temperature of the welding tip and support anvil during the welding cycles and controls their temperature to insure uniform weld size and strength over the entire welding period.